Modern turbomachines comprise a number of individual modules, each readily separable from its neighboring modules to facilitate maintenance and repair. The principal modules of a typical turbomachine include at least a low pressure compressor, a high pressure compressor, a combustor, a high pressure turbine and a low pressure turbine, all arranged in series along an axially extending centerline. Each compressor and turbine module includes a case that circumscribes one or more axially alternating arrays of blades and vanes. The blades are secured to a rotatable hub so that they project radially from the hub toward the case and span a working medium flowpath. The vanes extend radially inwardly from the case and, like the blades, span across the working medium flowpath. An array of blades and an axially adjacent array of vanes is referred to as a stage.
During operation the compressors pressurize a stream of working medium fluid, usually ambient air, impelling the air to flow into the combustor. A set of fuel injectors introduces fuel into the air stream to produce a fuel-air mixture that readily ignites and burns, releasing the energy content of the fuel. The resulting high pressure combustion products then partially expand through the turbines to drive the compressors. Finally, the partially expanded combustion products flow through a power converter where they expand further to ambient pressure. Typical examples of power converters include an exhaust nozzle that generates propulsive thrust for an aircraft or a power turbine for driving pumps, generators, propellers or other equipment requiring power and rotary motion.
The high pressure turbine operates in a hostile and demanding environment. The combustion products entering the high pressure turbine are extremely hot, sometimes exceeding 2500.degree. F. (1370.degree. C.), and highly corrosive. Moreover the combustion products flow through the turbine with a considerable velocity and may carry small particulates capable of eroding the surfaces of the turbine blades and vanes. Therefore, turbomachinery manufacturers specify a maximum service interval for the high pressure turbine module. At the expiration of the service interval, maintenance technicians separate the high pressure turbine module from the turbomachine, substantially disassemble the module, and refurbish or replace the blades, vanes and other internal components. The requirement to substantially disassemble the module is not a significant hardship since high pressure turbine modules are customarily compact, both axially and radially, and often contain only a single stage of blades and vanes. Moreover, complete disassembly is justifiable since nearly all the internal components require repair, replacement or at least a thorough inspection at the end of the specified service interval.
The low pressure turbine module, by contrast, operates in a less punishing environment since the temperature, corrosive potential and velocity of the combustion products diminish considerably as the combustion products flow through the high pressure turbine. Therefore, the specified service interval of a low pressure turbine is usually two to three times as long as that of a high pressure turbine. That is, complete disassembly and comprehensive service of a low pressure turbine is usually necessary only at the conclusion of every second or third high pressure turbine service interval. Nevertheless it may occasionally be necessary to conduct unscheduled service on selected internal components of a low pressure turbine prior to reaching the end of its service interval. The components most susceptible to damage, and therefore most likely to require unscheduled service, are the forwardmost components of the low pressure turbine module, usually an array of stator vanes.
Unfortunately low pressure turbine modules are not ordinarily configured for easy, convenient removal of the forwardmost vanes. The diameter of a typical low pressure turbine case increases appreciably between its forward and aft ends to accommodate expansion of the combustion products. Therefore, the module is normally designed so that the blade and vane arrays are installable and removable sequentially from the aft, larger diameter end of the module. Although this arrangement facilitates complete assembly and disassembly of the module, it is neither time efficient nor cost effective if only the forwardmost vanes require removal and service. This is particularly true since the low pressure turbine module is heavy, unwieldy and contains multiple stages of blades and vanes.
The need to repair or replace the forwardmost vanes prior to expiration of the specified service interval for the low pressure turbine module might be avoided by constructing the vanes of more exotic materials, by applying thermal barrier, erosion resistant and corrosion resistant coatings to the exposed surfaces of the vanes, or by cooling the vanes with relatively cool air diverted from one of the compressors. However all of these options are unappealing since they add weight, cost or complexity to the engine.
What is needed is a turbomachinery module whose damage susceptible vanes are removable and installable through a designated end of the module, and particularly a turbine module whose forwardmost vanes are conveniently removable and installable through the forward, small diameter end of the case.